Problem: $\dfrac{3}{100} + \dfrac{9}{10} = {?}$
Solution: The fractions must have the same denominator before you can add them. Rewrite $\dfrac{9}{10}$ as $\dfrac{90}{100}$ $\dfrac{3}{100} + \dfrac{90}{100} = {?}$ $ = \dfrac{93}{100}$